Graphene is a two-dimensional novel material composed of a single layer of sp2-hybridized carbon atoms in a honeycomb lattice. Graphene is known as a good conductor that is very stable structurally and chemically and carries electrons approximately 100 times faster than silicone and approximately 100 times higher current than copper does. Further, graphene is approximately 200 times stronger in mechanical strength and more flexible than steel and has a heat conductivity of about 5,300 W/m·K and a good optical property such as a light transmittance of 97.7%. Composed of carbon atoms alone, graphene has a vast specific surface area of 2,600 m2/g. Such characteristics of graphene enables the use of graphene for transparent electrodes, touch panels, flexible displays, high-sensitivity sensors, catalysts, and so forth and advantageously allows graphene in combination with another material to form a composite material. In recent years, graphene has emerged as a promising second-generation semiconductor material.
Currently, there are three different production methods for graphene: the mechanical exfoliation method, the chemical vapor deposition (CVD) growth, and the chemical exfoliation method. Depending on the production methods, graphenes have different properties and hence different applications. The mechanical exfoliation method is a method of using a mechanical force to exfoliate graphite as a base material of graphene into graphene flakes. The mechanical exfoliation method that uses a scotch tape to produce graphene flakes has been a great contribution to the early studies on graphene. The graphene produced by the mechanical exfoliation method is of a relatively high quality but disadvantageous in regards to poor applicability and difficulty of production on a large scale. The CVD growth makes the use of the fact that a metal catalyst such as of nickel or copper is capable of adsorbing carbon atoms at high temperature. Thus, the CVD growth is a method of exposing graphene to a metal catalyst and a methane/hydrogen gas mixture at high temperature of 1,000° C. to have carbon melt on the surface of the metal catalyst and then cooling down to crystallize the carbon atoms on the surface of the metal catalyst. The graphene produced by the CVD growth is of a high quality but needs to control the conditions, such as the type and thickness of the metal catalyst, the reaction time, the cooling rate, the concentration of the reaction gas, etc. Further, the fabrication of CVD-grown graphene thin film requires an additional process of transferring the produced graphene to a desired substrate. Most of all, the high-temperature condition of the process such as 1,000° C. is a huge obstacle to the industrial use of the CVD growth. The chemical exfoliation method is a method of using an oxidizing agent or a surfactant to chemically exfoliate graphite in a solution state into graphene flakes. The oxidized graphene flakes produced with an oxidizing agent can return to the graphene with a reducing agent such as hydrazine, etc. The chemical exfoliation method has a disadvantage that the graphene oxide flakes separated by oxidization are not completely reduced to remain many defects, ending up deteriorating the electrical properties of the graphene. But, the chemical exfoliation method involves relatively soft conditions, has easiness of graphene production on large scale and allows production of graphene into various structures such as graphene-based hydrogel particles or graphene paper as well as graphene thin film. Further, the graphene produced by the chemical exfoliation method is easy to form a composite with another material and uniquely distinctive from the other graphene production methods in the aspect of applicability. Therefore, many studies have been made to extend the fields of the applications of graphene using the chemical exfoliation method.
The most important technique required to use the graphene oxides produced by the chemical exfoliation method for such applications as transparent electrodes, touch panels, flexible displays, high-sensitivity sensors, etc. is to uniformly apply the graphene oxides existing in the solution state on a desired substrate.
Conventionally, the graphene oxide flakes in the solution state are fabricated in the form of film through various coating methods and self-assembling methods, such as spin coating, spray coating, vacuum filtration, Langmuir-Blodgett (LB) assembly, layer-by-layer (LBL) assembly, etc.
The spin coating is the most widely used coating method, where a predetermined amount of a graphene oxide solution is put dropwise on a substrate, which is rotated at high spinning rate to coat the substrate by the centrifugal force imposed on the solution. In the case of the spin coating, an extremely high spin speed causes a failure to combine the graphene oxide flakes closely together and thus deteriorates the quality of the film on the whole; whereas an extremely low spinning rate ends up with the film too thick to acquire a uniform coating, making the film difficult to use as a transparent electrode. Further, the spin coating has the difficulty in acquiring a uniform coating on a large-area substrate.
The spray coating, which is a coating method to spray a graphene oxide solution on a substrate, makes it easy to get a coating on a large-area substrate and involves a process fast and simple. But, the spray coating causes graphene oxide flakes to aggregate before the solution sprayed reaches the substrate, ending up producing a film not uniform in the whole area.
The vacuum filtration is a coating method to pass a graphene oxide solution through a fine filter paper and filter graphene oxide flakes out, forming a film with the graphene oxide flakes. The vacuum filtration makes it possible to obtain a uniformly coated film and easy to control the thickness of the film, but it consumes a large amount of the graphene oxide solution and takes a long time for the process. More disadvantageously, the vacuum filtration requires an additional process to transfer the film formed on the fine filter paper to a desired substrate.
The Langmuir-Blodgett (LB) assembly is a coating method to immerse a substrate vertically in a solution having graphene oxide flakes arranged on the surface and then slowly lift it at a constant rate so that the graphene oxide flakes can self-assemble on the substrate. The LB assembly forms a relatively uniform film but takes a long time for the process, with the difficulty to form a coating on a large-area substrate.
The lay-by-lay (LBL) assembly is a coating method to apply different surface charges on the graphene flakes and assemble a film using the static attraction. For the LBL assembly, functional groups are affixed to the graphene flakes to prepare a graphene oxide solution having the positive electric charge and a graphene oxide solution having the negative electric charge, and the substrate is immersed alternately in the two solutions to build up the graphene oxide flakes in a lay-by-lay manner. The LBL assembly offers good workability, but it needs a pretreatment process for affixing functional groups to the graphene flakes, uses a great amount of the graphene oxide solution and takes too much time.
Accordingly, in order to overcome the problems with the conventional coating methods for fabricating a graphene thin film using graphene oxides prepared by the chemical exfoliation method, there is a demand for a novel production method for graphene thin film that can fabricate a uniform and large-area graphene thin film for transparent electrode using a small amount of a graphene oxide solution in a short process time without limiting the area of the substrate.